One of the challenges in working on offshore operations is dealing with the constant motion due to the ocean's waves. The constant heaving and surging of the waves presents numerous challenges to those involved in the transfer of payloads from ships or platforms to positions on or below the ocean's surface.
In a typical lowering situation, the payload is first lifted off the deck of a ship using a winch having a cable running through a sheave rotatably mounted on an A-frame or crane boom. The crane or A-frame luffs overboard and the winch cable is paid out to lower the load. Once the payload touches the peaks of the waves, the ocean's influence causes relative motion between the ocean's surface and the object being moved. The relative motion engendered between the ocean's surface and the object being moved must be taken into account and compensated for to accurately deliver the payload. Movement of towed loads that travel close to the ocean floor represent a risky endeavor for many reasons, one of which is that large relative degrees of motion are induced into the towed load due to the ship's response to movement of the water's surface. Docking or maneuvering an object suspended from a ship's crane or other lifting device near fixed objects, in the ocean or on the ocean floor, is nearly impossible unless special means are taken to reduce or eliminate the relative motions. Additionally, when the relative motions are in excess of the load's terminal velocity in the water, snap loads occur in the lowering cable. These snap loads are dangerous to the survival of the cable, its terminations, and to the load and lifting device in general. Since these relative motions increase with increasing seas, the range of weather in which these lowering operations can be carried out is restricted. Since larger ships induce smaller motions, larger ships are often required for critical lowering operations.
Various heave compensation devices have been proposed in an effort to overcome these difficulties. These devices generally attempt to maintain the load in a more or less fixed position relative to the earth, regardless of the motions that the ship is undergoing by creating reciprocal movements in the lowering cable in an attempt to compensate for the relative motion. Control of these devices may be either passive or active, with relative expense, space and weight considerations being deciding factors in this regard. Various mechanisms have been utilized in attempting to raise and lower the required amount of cable to produce the reciprocal movements, including active winch drums, flying sheaves, and nodding booms.